There are many chemical vapor deposition techniques available which may be used for the formation of thin films on a substrate surface. The use of two or more reagents in the chemical vapor deposition of films is often required, and the stoichiometric ratio of the reagents which are employed is a very important feature in the production of high-quality thin films. However, many chemical vapor deposition techniques use volatile and semivolatile reagents as starting materials which react in the vapor phase or at the substrate surface to form a thin film on the surface. When separate sources of volatile reagents are used, the stoichiometry and homogeneity of the reagents at the substrate surface are often hard to control owing to inadequate mixing of the reagents and/or variations in the reagent vapor pressure at the respective reagent reservoirs, temperature fluctuations and reagent decomposition. Other techniques for forming thin films, for example sputtering, laser evaporation and electron beam evaporation, use nonvolatile sources but are disadvantageous in that they are extremely slow and energy inefficient. Thus, a need exists for deposition techniques for forming thin films which allow improved control of reagent stoichiometry and homogeneity and which are relatively fast and energy efficient in use.
The use of supercritical fluid solutions in the formation of thin films, powders and fibers is also known. For example, the Smith U.S. Pat. No. 4,582,731 discloses methods for solid film deposition and fine powder formation by dissolving a solid material in a supercritical fluid solution at an elevated pressure and then rapidly expanding the solution through an orifice into a region of relatively low pressure. The Smith reference discloses only physical processes such as solvent evaporation to form films as contrasted with the chemical processes used in the present Invention as will be discussed in detail below. Similar methods are disclosed by Peterson et al., Polymer Engineering and Science, Vol. 27, No. 22, pp. 1693-1697 (1987), and Matson et al., Industrial and Engineering Chemical Research, Vol. 26, pp. 2298-2306 (1987). However, these methods are disadvantageous in that the deposited material must be soluble in the supercritical fluid. Thus, the previously known methods employing supercritical fluids do not solve the deficiencies of chemical vapor deposition methods noted above.